Calorimetric study of the thermodynamic properties of Mn ₅ O ₈

Manganese oxides occur widely in nature and have technical applications in various areas. This study quantitatively evaluates the thermodynamic properties of Mn ₅ O ₈ , a binary manganese oxide that has a layered structure and contains coexisting divalent and tetravalent manganese. Three samples of the Mn ₅ O ₈ phase with slightly different manganese average oxidation states (Mn AOS) were synthesized using a wet chemical method and annealing. Synchrotron X-ray analysis revealed that the samples contain a small amount of a secondary MnO ₂ phase that cannot be identified using laboratory X-ray diffraction. High-temperature oxide melt solution calorimetry in molten sodium molybdate at 700°C showed that all three samples are slightly higher in enthalpy than an isochemical mixture of bixbyite (Mn ₂ O ₃ ) and pyrolusite (MnO ₂ ), probably rendering them metastable in free energy with respect to isochemical mixtures of bixbyite and pyrolusite. However, the energetic metastability (endothermic enthalpy) of Mn ₅ O ₈ is very small (<6 kJ/mol) and does not depend significantly on the Mn AOS. Thus, although Mn ₅ O ₈ probably does not appear on the equilibrium Mn-O phase diagram, its small metastability allows its synthesis by a variety of low temperature reactions.